No you have to go up in frequency until the wavelength is significantly shorter than the spaces between the nucleii which means pretty hard Xrays before copper and most other conductore become significantly transparent Then you do not have a conventional refractive index only a diffraction pattern off the reasonably regularly spaced nucleii

lyner

It will always be diffracted - the degree of diffraction will depend upon the geometry - aperture and molecular dimensions. In a 'block' of glass the aperture is enormous so the 'ray' is relatively unaffected in many cases. If you are using a glass telescope lens, diffraction is very relevant; it limits its resolution. Atoms are smaller than light wavelengths so they diffract the energy they intercept in all directions (scattering). The resultant wave pattern produces an undeviated ray - but diffraction must still have taken place.The refraction will depend upon the electronic energy levels etc etc in the substance, which affects the phases and amplitudes of photons when they are absorbed and re radiated by the atoms. This affects the speed of the wave.

Well, I could throw a spanner in here and say that light always travels at c and that what we are in effect seeing is an absorption/re-emission delay.If the energy of the photon corresponds exactly to the energy needed to 'bump' an electron to a higher shell then we have absorption. If it is, instead, close but not exact then we get a 'virtual' bump followed by a re-emission of the photon a small time later, as the electron returns to its normal 'shell'. The closer the energy match, the more the time for the re-emission. The amount of time taken for the re-emission is a measure of the refractive index.

(This is a semi-classical, semi-quantum explanation. For a full quantum explanation then you need a bigger brain than mine :-) )

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lyner

I'd go with most of that except for your use of "shell". In a solid there aren' t discrete levels but bands (Pauli applies) and any frequency of wave can interact. The density will affect the delay because of the number of interactions per cm. In a gas the only 'simple' electron interactions can be at spectral lines. I think there must be some other effect to account for speed change - vibrating the whole atom, perhaps ?( neutral charge tho so that sounds suspect) If it were frequency dependent there would be strong dispersion. I'll have to read round.

This question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower than it is in vacuum. This FAQ will strictly deal with that scenario only and will not address light transport in anomolous medium, atomic vapor, metals, etc., and will only consider light within the visible range.

The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics). So this explanation will attempt to only provide a very general and rough idea of the process.

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.

Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other. Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".

When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.>>__________________

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lyner

That's nicely put. 'The Hydrogen Atom' model which we all get taught is all too often used out of context and the Energy Levels in solids are much more complicated.The quotation doesn't deal with the behaviour of light passing through a gas, though. Gases have very low refractive indices but the effect is still very broad band (except at absorption peaks). I wonder how that can be explained, in similar sorts of terms; it can't be due to mechanical vibrations - can it?

Having said that, the effect of em waves through ionised media can be treated as forced oscillations of the free electrons in a classical way.

When light passes through a glass prism it breaks down to it's rainbow colours. Why when light passes through a glass block, it does not break down to it's rainbow colours?

Because light passing through a prism is forced to refract in the same direction when it enters and when it escapes; passing through a glass block no: it is refracted in a direction when it enters and refracted in the opposite direction when it escapes, so the total refraction is zero.

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